In an assessment of the performance of optical transmission systems, bit error rate (BER) tests are usually used. BER is defined as the ratio between the number of erroneously received bits to the total number of bits received over a period of time. In modern optical transmission systems, the BER test normally takes a long time to perform. For example, to evaluate a BER of 10−14 for data that is transmitted at a bit rate of 2.5 Gb/s, the measurement time needed is 12 hours. Performance of an optical system can also be defined by a parameter called Q-factor. The Q-factor indicates the signal-to-noise ratio of the signal and is defined as:
  Q  =                    μ        1            -              μ        0                            σ        1            +              σ        0            where μ1 is the mean value of the “1's”, μ0 is the mean value of the “0's”, σ1 is the standard deviation of the level of “1's” and σ0 is the standard deviation of the level of “0's”. Q-factor measurement can greatly accelerate the test. Through reducing test time, the efficiency and benefit in cost and time can be obtained in design, manufacture, installation, maintenance and monitor of optical transmission systems.
Several methods have been proposed to estimate the BER by calculation of the Q-factor. For example, one method disclosed in an article entitled “Margin Measurements in Optical Amplifier Systems” by Neal S. Bergano, F. W. Kerfoot and C. R. Davidson, published in IEEE Photonics Technology Letters, Vol. 5, No. 3, March 1993, adjusts the ‘Decision Threshold’ level of a tester's receiver away from the optimal value which gives the minimum BER. The shift of the decision threshold level increases the BER measured to a high level that is measurable in a short time. The measured high BER values are then used to mathematically extrapolate to the BER at the optimal decision threshold.
Another known method is the ‘Light Interference’ method, which was described by P. Palacharla, J. Chrostowski and R. Neumann in a paper entitled “Techniques for Accelerated Measurement of Low Bit Error Rates in Computer Data Links” published in the Proceedings of the IEEE Fourteenth Annual International Phoenix Conference on Computers and Communications, Scottsdale, Ariz., Mar. 28-31, 1995, pp.184-190. In this method, a sinusoidal interfering light source is coupled to the transmission data signal to increase the BER measured at the receiver, allowing the high BER to be measured in a short time. Through the resultant Q-factor measurement, BER in the absence of the interference signal can then be extrapolated.